1. Field of the Invention
The present invention relates to a combustion/misfire detecting apparatus for an internal combustion engine for detecting combustion and misfire events taking place within a cylinder of the engine on the basis of an ion current making appearance upon combustion of an air/fuel mixture within the cylinder.
2. Description of Related Art
In general, it is known that ions are produced when the air/fuel mixture is burned within a cylinder of an internal combustion engine (hereinafter also referred to simply as the engine). Such ions can be detected in the form of an ion current by means of a probe which is disposed within the cylinder and to which a high voltage is applied as a bias voltage. Thus, by detecting presence or absence of the ion current, it is possible to detect occurrence of combustion or misfire event within all the engine cylinders discriminatively from one another.
For having better understanding of the concept underlying the present invention, background techniques thereof will first be reviewed in some detail. FIG. 11 is a block diagram showing a structure of a conventional misfire detecting apparatus for an internal combustion engine. In the figure, reference numeral 1 denotes an ignition circuit comprised of an ignition coil IG whose primary winding 11 has a high-voltage end applied with a voltage VB of positive or plus polarity while a low voltage end of the primary winding 11 is connected to a switching element 13 for turning on/off a primary current flowing through the primary winding 11. More specifically, the switching element is constituted by a power transistor having a collector electrode connected to the primary winding 11 as mentioned above and an emitter electrode connected to the ground potential. The base of the switching element 13 is connected to an input terminal of the ignition circuit 1 to which an ignition pulse signal IB issued by an electronic control unit or ECU (not shown) which is known per se is applied. On the other hand, the secondary winding 12 of the ignition coil IG has a high-voltage end connected to a spark plug 14 while the low-voltage end of the secondary winding 12 is connected to an ion current detecting unit 15 by a wiring conductor.
The ion current detecting unit 15 in turn is comprised of a bias circuit 16 for applying a vias voltage VB of plus polarity to the spark plug 14, a mask circuit 17 for cutting off or eliminating noises generated upon ignition and firing of the air/fuel mixture from the ion current as detected and a waveform shaper circuit 18 which is designed for shaping the ion current from which the noise component has been eliminated, to thereby output a combustion pulse signal. Incidentally, the noise generated upon firing of the air/fuel mixture as mentioned above will hereinafter be referred to as the ignition noise only for convenience of the description.
Next, description will be directed to the operation of the conventional misfire detecting apparatus. For making it possible to detect the ion current, the bias circuit 16 applies a high voltage of positive or plus polarity (also referred to as the bias voltage) to the spark plug 14 designed to serve also as an ion current detecting probe by making use of the secondary voltage of the ignition coil IG.
Upon application of an ignition pulse IB to the switching element 13, the primary current flowing through the primary winding 11 of the ignition coil IG is interrupted at a falling edge of the ignition pulse IB, as a result of which a high voltage of negative or minus polarity is applied to the spark plug 14 connected electrically to the secondary winding 12 of the ignition coil IG, whereby a spark discharge is caused to occur between the electrodes of the spark plug 14. Thus, the air/fuel mixture is fired to undergo explosive combustion, which results in generation of ions within the engine cylinder due to the effect of the ionization.
In that case, the spark plug 14 will continue to remain in the state applied with the bias voltage of plus polarity from the bias circuit 16 which is charged with the secondary voltage of the ignition coil, even after extinction of the spark discharge. Consequently, the ions produced due to the ionization are caused to migrate under the action of the bias voltage. This migration of ions is detected as an ion current.
In this conjunction, it is however noted that before the ion current is detected, a steep pulse P1 makes appearance in response to rising of the ignition pulse IB and additionally a steep pulse P3 is produced before generation of the ion current at the time point when the air/fuel mixture is fired by the spark discharge occurring at the spark plug 14, as is illustrated in FIG. 12 at (a) and (b). These pulses P1 and P3 are detected as the ignition noises as well.
In general, the peak value of the ion current changes in dependence on the operation state of the engine. More specifically, there exists such a trend that the peak value of the ion current becomes smaller as the rotation number or speed (rpm) of the engine decreases while the former becomes larger as the latter increases. Usually, the peak value of the ion current lies within a range of several microamperes (xcexcA) to several hundred microamperes. Under the circumstances, the threshold value used for detecting the ion current is set on the order of several microamperes with a view to detecting occurrence of the misfire event on the basis of the presence/absence of the ion current over the whole operation range of the engine.
However, when the threshold value is set on the order of several microamperes in practical applications as mentioned above, there may arise such unwanted situation that the ignition noises P1 and P3 produced upon rising of the ignition pulse IB as well as upon occurrence of the spark discharge at the spark plug 14 will be detected erroneously as the combustion pulse (i.e., the pulse indicating the combustion of the air/fuel mixture). For this reason, the steep noise pulses P1 and P3 each of a very short duration are eliminated by the mask circuit 17 so that only the ion current component is shaped into a pulse signal by the waveform shaper circuit 18 to be thereby outputted as the combustion pulse signal. Thus, so long as the combustion of the air/fuel mixture occurs normally, the combustion pulse signal indicative of the ion current can be outputted from the bias circuit 16 after lapse of the masking period from the start of the spark discharge, as can be seen in FIG. 12 at (c).
Referring to FIG. 12 at (d), there is illustrated a waveform of the ion current signal outputted upon occurrence of a misfire event. As can be seen from this figure, the noise pulse P1 generated upon rising of the ignition pulse as well as the noise pulse P3 generated upon occurrence of the spark discharge make appearance as the ignition noises. However, these ignition noise pulses P1 and P3 are eliminated by the mask circuit 17. Of course, no combustion pulse originating in the ion current is outputted from the waveform shaper circuit 18 either, because no combustion/explosion has taken place within the cylinder (i.e., because of occurrence of the misfire event), as is illustrated in FIG. 12 at (e).
So long as the normal combustion of the air/fuel mixture takes place, it is thus possible to make decision as to occurrence of the combustion/misfire events on the basis of presence/absence of the combustion pulse which can be derived by shaping the ion current with reference to a fixed threshold value. In this conjunction, it is however to be noted that soot may be deposited on the electrodes of the spark plug 14 as well as the inter-electrode gap thereof due to repetitive combustion of the air/fuel mixture although it depends on the operation state of the internal combustion engine. Such deposition of soot gives rise to a problem that a leak current takes place.
In more concrete, it is assumed, by way of example, that the bias voltage is 100 V and that the insulation resistance of the spark plug 14 deposited with soot is 5 Mxcexa9. Then, the leak current of 20 xcexcA can flow. More specifically, the leak current of 20 xcexcA which attenuates monotonously with a predetermined time constant will flow into the ion current detecting unit 15, starting from the time point when the ignition pulse IB is applied, as can be seen in FIG. 13 at (a) and (b). Furthermore, in succession to the start of spark discharge at the spark plug 14, the leak current which decreases gradually and monotonously with the time constant CR determined by the high resistance presented by the soot deposition and the capacitance component C of the bias circuit makes appearance, wherein the ion current originating in the combustion of air/fuel mixture is superposed on the leak current, as is indicated by hatching in FIG. 13.
As can now be appreciated, when the leak current flows take place as mentioned above, then pulse shaping of the ion current inputted through the mask circuit 17 by the waveform shaper circuit 18 will incur a problem that the leak current of a predetermined pulse width produced upon application of the ignition pulse as well as the leak current making appearance upon spark discharge and decreasing monotonously and gradually with the time constant CR may undesirably be detected as the combustion pulses regardless of occurrence or non-occurrence of the misfire event. In other words, occurrence of the misfire event can not be detected with reasonable reliability when the leaks current happen.
In the light of the state of the art described above, it is an object of the present invention to provide a misfire detecting apparatus for an internal combustion engine which apparatus is capable of detecting only the ion current component originating in the fuel combustion regardless of occurrence of the leak currents so long as the combustion takes place normally, to thereby solve the problem mentioned above.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to an aspect of the present invention an misfire detecting apparatus for an internal combustion engine for detecting combustion of an air/fuel mixture and occurrence of misfire event within an engine cylinder of an internal combustion engine on the basis of an ion current which makes appearance upon combustion of the air/fuel mixture. The misfire detecting apparatus includes an ion current detecting means for detecting an ion current which depends on an amount of ions generated within the engine cylinder immediately after combustion of the air/fuel mixture within the engine cylinder, a leak current detecting means for detecting a leak current occurring between electrodes of a spark plug for firing the air/fuel mixture within the engine cylinder, a first waveform shaping means for shaping the ion current as detected into a pulse signal indicating combustion/misfire event by comparing with a first threshold value for deciding the ion current, the first threshold value being set on the basis of a level of the ion current upon rising thereof, the ion current being superposed on the leak current, and a means for outputting the pulse signal as a combustion pulse signal indicating combustion of the air/fuel mixture and occurrence of misfire event.
By virtue of the arrangement of the misfire detecting apparatus described above, the ion current superposed on the leak current can be shaped into a combustion pulse regardless of magnitude of the leak current, whereby the combustion pulse signal indicating occurrence of the fuel combustion within the relevant engine cylinder can be obtained with high reliability.
In a preferred mode for carrying out the invention, the misfire detecting apparatus for an internal combustion engine may further include a second waveform shaping means for shaping the ion current as detected into a pulse signal through comparison with a second fixed threshold value which is fixed at a value smaller than the first threshold value mentioned previously, and a selecting means for selectively outputting as the combustion pulse signal either a shaped pulse signal outputted from the first waveform shaping means upon detection of occurrence of the leak current or a shaped pulse signal outputted from the second waveform shaping means when no leak current is detected.
Owing to the misfire detecting apparatus described above, the combustion or misfire event can be detected with enhanced reliability notwithstanding of occurrence of the leak current, to an advantage.
In another preferred mode for carrying out the invention, the leak current detecting means may be so arranged as to compare a level of the ion current detected by the ion current detecting means during every combustion stroke of the internal combustion engine with a preset fixed threshold value for shaping the ion current into a pulse signal to thereby decide occurrence of the leak current when an integrated value of a predetermined current charged and discharged at high and low levels, respectively, of the pulse signal exceeds a predetermined value inclusive thereof.
Owing to the arrangement described above, the combustion and the misfire event can be detected with high reliability independently from operation state of the internal combustion engine, to another advantage.
In yet another preferred mode for carrying out the invention, the leak current detecting means may be so arranged as to decide occurrence of the leak current when a pulse width of a pulse signal derived by shaping the ion current signal detected by the ion current detecting means during an output period of an ignition pulse applied to the spark plug becomes greater a predetermined value inclusive thereof.
With the arrangement described above, the combustion and the misfire event can be detected at an earlier time point, to a further advantage.
In a further preferred mode for carrying out the invention, the leak current detecting means may be so arranged that upon power-on of the apparatus, the selecting means selects the pulse signal outputted from the first waveform shaping means, while in succeeding combustion strokes, occurrence of the leak current is detected in dependence on a pulse duration of the pulse signal outputted from the second waveform shaping means, to thereby allow the selecting means to change over the waveform shaping means.
With the above-mentioned arrangement, the combustion and the misfire event can be detected with high accuracy and reliability immediately after the start of the misfire detecting apparatus, to still another advantage.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.